Volume-time profile during relaxed expiration in the normal dog

Electropneumatic system for the simulation of the pulmonary viscoelastic effect in a mechanical ventilation scenario

The viscoelastic properties of the lung have important implications during respiratory mechanics in terms of lung movement or work of breathing, for example. However, this property has not been well characterized due to several reasons, such as the complex nature of the lung, difficulty accessing its tissues, and the lack of physical simulators that represent viscoelastic effects. This research proposes an electropneumatic system and a method to simulate the viscoelastic effect from temporary forces generated by the opposition of magnetic poles. The study was tested in a mechanical ventilation scenario with inspiratory pause, using a Hamilton-S1 mechanical ventilator (Hamilton Medical) and a simulator of the human respiratory system (SAMI-SII). The implemented system was able to simulate the stress relaxation response of a Standard Linear Solid model in the Maxwell form and showed the capacity to control elastic and viscous parameters independently. To the best of our knowledge, this is the first system incorporated into a physical lung simulator that represents the viscoelastic effect in a mechanical ventilation scenario.

How I Teach Auto-PEEP: Applying the Physiology of Expiration

Teaching complex topics in mechanical ventilation can prove challenging for clinical educators, both at the bedside and in the classroom setting. Some of these topics, such as the topic of auto-positive end-expiratory pressure (auto-PEEP), consist of complicated physiological principles that can be difficult to convey in an organized and intuitive manner. In this entry of "How I Teach," we provide an approach to teaching the concept of auto-PEEP to senior residents and fellows working in the intensive care unit. We offer a framework for educators to effectively present the concepts of auto-PEEP to learners, either at the bedside or in the classroom setting, by summarizing key concepts and including concrete examples of the educational techniques we use. This framework includes specific content we emphasize, how to present this content using a variety of educational resources, assessing learner understanding, and how to modify the topic on the basis of location, time, or resource constraints.

Standard and viscoelastic mechanical properties of respiratory system compartments in dogs: Effect of volume, posture, and shape

In 9 anesthetized, paralyzed dogs lung and chest-wall standard (viscous resistance, Rint, and quasi-static elastance, Est) and viscoelastic parameters (resistance, Rvel, and time constant, τvel) were measured in the supine posture before and after rib-cage block, after application of an expiratory threshold load, and after 75° head-up tilting before and after wide chest opening. Lung and chest-wall τvel were the same under all conditions. Rvel was independent of volume and posture, and greater for the lung. Chest-wall Rint was independent of flow, volume, and posture. Lung Rint decreased with increasing volume. Chest-wall Rint, Est and Rvel increased with rib-cage block, allowing the assessment of both abdominal-wall and rib-cage characteristics. When chest opening did not elicit bronchoconstriction, the decrease of Rvel was ∼6%. Main conclusions: lung and chest-wall exhibit linear tissue viscoelasticity within the range studied; rib-cage and abdomen characteristics are similar, and asynchronous motion is not expected at physiological respiratory rates; in normal lungs, heterogeneity of parallel time constants plays a marginal role.

Bosutinib Therapy Ameliorates Lung Inflammation and Fibrosis in Experimental Silicosis

Silicosis is an occupational lung disease for which no effective therapy exists. We hypothesized that bosutinib, a tyrosine kinase inhibitor, might ameliorate inflammatory responses, attenuate pulmonary fibrosis, and thus improve lung function in experimental silicosis. For this purpose, we investigated the potential efficacy of bosutinib in the treatment of experimental silicosis induced in C57BL/6 mice by intratracheal administration of silica particles. After 15 days, once disease was established, animals were randomly assigned to receive DMSO or bosutinib (1 mg/kg/dose in 0.1 mL 1% DMSO) by oral gavage, twice daily for 14 days. On day 30, lung mechanics and morphometry, total and differential cell count in alveolar septa and granuloma, levels of interleukin (IL)-1β, tumor necrosis factor (TNF)-α, interferon (IFN)-γ, IL-4, transforming growth factor (TGF)-β, and vascular endothelial growth factor in lung homogenate, M1 and M2 macrophages, total leukocytes, and T cells in BALF, lymph nodes, and thymus, and collagen fiber content in alveolar septa and granuloma were analyzed. In a separate in vitro experiment, RAW264.7 macrophages were exposed to silica particles in the presence or absence of bosutinib. After 24 h, gene expressions of arginase-1, IL-10, IL-12, inducible nitric oxide synthase (iNOS), metalloproteinase (MMP)-9, tissue inhibitor of metalloproteinase (TIMP)-1, and caspase-3 were evaluated. In vivo, in silicotic animals, bosutinib, compared to DMSO, decreased: (1) fraction area of collapsed alveoli, (2) size and number of granulomas, and mononuclear cell granuloma infiltration; (3) IL-1β, TNF-α, IFN-γ, and TGF-β levels in lung homogenates, (4) collagen fiber content in lung parenchyma, and (5) viscoelastic pressure and static lung elastance. Bosutinib also reduced M1 cell counts while increasing M2 macrophage population in both lung parenchyma and granulomas. Total leukocyte, regulatory T, CD4⁺, and CD8⁺ cell counts in the lung-draining lymph nodes also decreased with bosutinib therapy without affecting thymus cellularity. In vitro, bosutinib led to a decrease in IL-12 and iNOS and increase in IL-10, arginase-1, MMP-9, and TIMP-1. In conclusion, in the current model of silicosis, bosutinib therapy yielded beneficial effects on lung inflammation and remodeling, therefore resulting in lung mechanics improvement. Bosutinib may hold promise for silicosis; however, further studies are required.

Dasatinib Reduces Lung Inflammation and Fibrosis in Acute Experimental Silicosis

Silicosis is an occupational lung disease with no effective treatment. We hypothesized that dasatinib, a tyrosine kinase inhibitor, might exhibit therapeutic efficacy in silica-induced pulmonary fibrosis. Silicosis was induced in C57BL/6 mice by a single intratracheal administration of silica particles, whereas the control group received saline. After 14 days, when the disease was already established, animals were randomly assigned to receive DMSO or dasatinib (1 mg/kg) by oral gavage, twice daily, for 14 days. On day 28, lung morphofunction, inflammation, and remodeling were investigated. RAW 264.7 cells (a macrophage cell line) were incubated with silica particles, followed by treatment or not with dasatinib, and evaluated for macrophage polarization. On day 28, dasatinib improved lung mechanics, increased M2 macrophage counts in lung parenchyma and granuloma, and was associated with reduction of fraction area of granuloma, fraction area of collapsed alveoli, protein levels of tumor necrosis factor-α, interleukin-1β, transforming growth factor-β, and reduced neutrophils, M1 macrophages, and collagen fiber content in lung tissue and granuloma in silicotic animals. Additionally, dasatinib reduced expression of iNOS and increased expression of arginase and metalloproteinase-9 in silicotic macrophages. Dasatinib was effective at inducing macrophage polarization toward the M2 phenotype and reducing lung inflammation and fibrosis, thus improving lung mechanics in a murine model of acute silicosis.

Expanded endothelial progenitor cells mitigate lung injury in septic mice

Endothelial progenitor cells (EPCs) improve survival and reduce organ failure in cecal ligation and puncture-induced sepsis; however, expanded EPCs may represent an even better approach for vascular repair. To date, no study has compared the effects of non-expanded EPCs (EPC-NEXP) with those of expanded EPCs (EPC-EXP) and mesenchymal stromal cells of human (MSC-HUMAN) and mouse (MSC-MICE) origin in experimental sepsis. One day after cecal ligation and puncture sepsis induction, BALB/c mice were randomized to receive saline, EPC-EXP, EPC-NEXP, MSC-HUMAN or MSC-MICE (1 × 10⁵) intravenously. EPC-EXP, EPC-NEXP, MSC-HUMAN, and MSC-MICE displayed differences in phenotypic characterization. On days 1 and 3, cecal ligation and puncture mice showed decreased survival rate, and increased elastance, diffuse alveolar damage, and levels of interleukin (IL)-1β, IL-6, IL-10, tumor necrosis factor-α, vascular endothelial growth factor, and platelet-derived growth factor in lung tissue. EPC-EXP and MSC-HUMAN had reduced elastance, diffuse alveolar damage, and platelet-derived growth factor compared to no-cell treatment. Tumor necrosis factor-α levels decreased in the EPC-EXP, MSC-HUMAN, and MSC-MICE groups. IL-1β levels decreased in the EPC-EXP group, while IL-10 decreased in the MSC-MICE. IL-6 levels decreased both in the EPC-EXP and MSC-MICE groups. Vascular endothelial growth factor levels were reduced regardless of therapy. In conclusion, EPC-EXP and MSC-HUMAN yielded better lung function and reduced histologic damage in septic mice.

Therapeutic effects of LASSBio-596 in an elastase-induced mouse model of emphysema

Emphysema is an intractable pulmonary disease characterized by an inflammatory process of the airways and lung parenchyma and ongoing remodeling process in an attempt to restore lung structure. There is no effective drug therapy that regenerates lung tissue or prevents the progression of emphysema; current treatment is aimed at symptomatic relief. We hypothesized that LASSBio-596, a molecule with potent anti-inflammatory and immunomodulatory effects, might reduce pulmonary inflammation and remodeling and thus improve lung function in experimental emphysema. Emphysema was induced in BALB/c mice by intratracheal administration of porcine pancreatic elastase (0.1 IU) once weekly during 4 weeks. A control group received saline using the same protocol. After the last instillation of saline or elastase, dimethyl sulfoxide, or LASSBio-596 were administered intraperitoneally, once daily for 8 days. After 24 h, in elastase-induced emphysema animals, LASSBio-596 yielded: (1) decreased mean linear intercept, hyperinflation and collagen fiber content, (2) increased elastic fiber content, (3) reduced number of M1 macrophages, (4) decreased tumor necrosis factor-α, interleukin-1β, interleukin-6, and transforming growth factor-β protein levels in lung tissue, and increased vascular endothelial growth factor. These changes resulted in increased static lung elastance. In conclusion, LASSBio-596 therapy reduced lung inflammation, airspace enlargement, and small airway wall remodeling, thus improving lung function, in this animal model of elastase-induced emphysema.

Infusion of bone marrow mononuclear cells reduces lung fibrosis but not inflammation in the late stages of murine silicosis

We hypothesized that infusion of bone marrow mononuclear cells (BMMCs) in the late stages of silica-induced damage would reduce the remodelling process in a murine model of silicosis. C57BL/6 mice were assigned to 2 groups. In the SIL group, mice were instilled with a silica particle suspension intratracheally. Control (C) mice received saline under the same protocol. On the 40th day, some of the animals from both groups were killed. The others were treated with either saline or BMMCs (1×10⁶cells) intravenously (C+BMMC and SIL+BMMC), and the mice were killed 70 days after the start of the protocol. In the mice in the SIL+BMMC group, collagen deposition, the presence of silica particles inside nodules, the presence of macrophages and cells reactive for inducible nitric oxide synthase were reduced. Lung parameters also improved. Beyond that, the total and differential cellularity of bronchoalveolar lavage fluid, immunoexpression of transforming growth factor-β, the number of T regulatory cells and apoptosis were increased. However, the presence of male donor cells in lung tissue was not observed using GFP+ cells (40d) or Y chromosome DNA (70d). Therefore, BMMC therapy in the late stages of experimental silicosis improved lung function by diminishing fibrosis but inflammatory cells persisted, which could be related to expansion of T regulatory cells, responsible for the beneficial effects of cell therapy.

Oleanolic acid improves pulmonary morphofunctional parameters in experimental sepsis by modulating oxidative and apoptotic processes

We compared the effects of oleanolic acid (OA) vs. dexamethasone on lung mechanics and histology, inflammation, and apoptosis in lung and distal organs in experimental sepsis. Seventy-eight BALB/c mice were randomly divided into two groups. Sepsis was induced by cecal ligation and puncture, while the control group underwent sham surgery. 1h after surgery, all animals were further randomized to receive saline (SAL), OA and dexamethasone (DEXA) intraperitoneally. Both OA and DEXA improved lung mechanics and histology, which were associated with fewer lung neutrophils and less cell apoptosis in lung, liver, and kidney than SAL. However, only animals in the DEXA group had lower levels of interleukin (IL)-6 and KC (murine analog of IL-8) in bronchoalveolar lavage fluid than SAL animals. Conversely, OA was associated with lower inducible nitric oxide synthase expression and higher superoxide dismutase than DEXA. In the experimental sepsis model employed herein, OA and DEXA reduced lung damage and distal organ apoptosis through distinct anti-inflammatory mechanisms.

Respiratory system dynamical mechanical properties: modeling in time and frequency domain

The mechanical properties of the respiratory system are important determinants of its function and can be severely compromised in disease. The assessment of respiratory system mechanical properties is thus essential in the management of some disorders as well as in the evaluation of respiratory system adaptations in response to an acute or chronic process. Most often, lungs and chest wall are treated as a linear dynamic system that can be expressed with differential equations, allowing determination of the system's parameters, which will reflect the mechanical properties. However, different models that encompass nonlinear characteristics and also multicompartments have been used in several approaches and most specifically in mechanically ventilated patients with acute lung injury. Additionally, the input impedance over a range of frequencies can be assessed with a convenient excitation method allowing the identification of the mechanical characteristics of the central and peripheral airways as well as lung periphery impedance. With the evolution of computational power, the airway pressure and flow can be recorded and stored for hours, and hence continuous monitoring of the respiratory system mechanical properties is already available in some mechanical ventilators. This review aims to describe some of the most frequently used models for the assessment of the respiratory system mechanical properties in both time and frequency domain.

On the interaction between respiratory compartments during passive expiration in ARDS patients

Relaxed expiratory volume-time profile has been frequently analysed by fitting exponential functions of time to one- or two-compartment models. In the latter case, the two exponential constants are assumed as representing the time constants of both compartments. Least-square fittings on the experimental data of five consecutive mechanically ventilated supine patients with acute respiratory distress syndrome (ARDS) were performed using rate-constants (flow/volume ratio) as parameters in order to obtain the model matching. Passive expiratory volume-time curves were recorded under PEEP = 0 and 13.6 +/- 3.3 S.D. cmH2O conditions. Model matching was optimal with significant, reliable parameter values. As a result, the use of a PEEP in ARDS patients: (a) delayed expiration; (b) decreased the percentage initial volume contribution of the slow-emptying compartment; and, (c) modified the interaction between compartments. The volume-time profile of the second compartment was found to increase at the beginning of expiration, and, then, progressively decayed towards zero, showing a maximum, although the overall curve decreased throughout expiration.

Determination of rate-constants as a method to describe passive expiration

To describe the relaxed expiration by a two-compartment model, we introduced a gas/energy transfer between the lung compartment ( V(1)) and a second one ( V(2)). If V(2) were a real volume, the rate-constants (i.e. the flow/volume ratios) of the compartments would describe a real gas-exchange. Alternatively, if a viscoelastic behaviour of the lung or an energy-exchange between compartments was simulated, V(2) would become a "pseudo-volume". We studied nine mechanically ventilated subjects. Changes in volume were transduced by respiratory inductive plethysmography. The rate-constants were assumed (together with the initial volumes of the compartments) as parameters to fit the total volume [ V(1)( t)+ V(2)( t)]. Once the best fitting was performed using these "physiological" parameters, the system was directly identified and the compartments were independently analysed. The time profile of the second compartment showed a maximum that depended on the value of the rate-constants. Appropriate tests confirmed the reliability of our procedure. In conclusion, our analysis demonstrated that the energy/volume of the second compartment may increase at the beginning of expiration and then decrease, showing a maximum, even though the total curve can only be a decreasing one. In other words, the slowing down of the curve representing expiratory volume is due not only to the longer emptying of the second compartment, but also to the interaction between the two compartments. As presently proposed, this interaction can be represented by either a gas exchange between two actual volumes, or a mechanical energy transfer between the lung and the tissue compartment.

Endotracheal cuff pressure as an index of airway smooth muscle activity: comparison with total lung resistance

Pressure changes in the cuff of an endotracheal tube (Pcuff) were measured as an index of the tracheal smooth muscle activity and compared with total lung resistance (RL) in anesthetized, paralyzed and artificially ventilated dogs. After obtaining passive pressure-volume relationships of the cuff in situ, we activated the airway smooth muscle by electrical stimulation of the right vagus nerve, intravenous acetylcholine, and airway mechanical stimulation. The responses elicited by vagal stimulation and airway probing affected predominantly the tracheal smooth muscle, whereas acetylcholine administration caused homogeneous responses in Pcuff and RL, suggesting involvement of the smooth muscle of the entire airway. Pcuff cannot represent the whole airway smooth muscle activity, but it is more sensitive than RL for detecting vagally mediated smooth muscle responses. We conclude that the combination of Pcuff and RL may provide a better evaluation of smooth muscle response to various stimuli.

Simple method to measure total expiratory time constant based on the passive expiratory flow-volume curve

OBJECTIVE

In intubated, mechanically ventilated patients, inspiration is forced by externally applied positive pressure. In contrast, exhalation is passive and depends on the time constant of the total respiratory system. The expiratory time constant is thus an important determinant of mechanical ventilation. The aim of this study was to evaluate a simple method for measuring the expiratory time constant in ventilated subjects.

DESIGN

Prospective study using a lung simulator and ten dogs.

SETTING

University hospital.

SUBJECTS

Commercially available lung simulator and ten greyhound dogs.

INTERVENTIONS

Different expiratory time constants were set on the lung simulator. In the dogs, the endotracheal tube was clamped to increase airways resistance by 22.5 cm H2O/(L/sec) and the lungs were injured with hydrochloric acid to decrease total respiratory compliance by 16 mL/cm H2O. This procedure resulted in a wide range of expiratory time constants.

MEASUREMENTS AND MAIN RESULTS

Pneumotachography was used to measure flow and volume. The ratio of exhaled volume and peak flow was calculated from these signals, corrected for the limited exhalation time yielding the "calculated expiratory time constant" and compared with the actual expiratory time constant. The typical error was +/- 0.19 sec for the lung simulator and +/- 0.15 sec for the dogs.

CONCLUSIONS

The volume and peak flow corrected for limited exhalation time is a good estimate of the total expiratory time constant in passive subjects and may be useful for the titration of mechanical ventilation.

A synthesis of the Otis, Mead, and Mount mechanical respiratory models

We propose a unified approach of the viscoelastic models introduced by Otis, Mead and Mount to describe the mechanical behaviour of the respiratory system. A single equation of motion is developed for these two-compartment models and solved in response to typical pressure and volume inputs. The analytic expression of the pressure response to rapid airway occlusion following constant flow inflation, and of the volume response to single breath following end-inspiratory pause are more particularly analysed. The lumped respiratory parameters derived from these responses are expressed as functions of the parameters characterising the constitutive mechanical elements of each model. Different combinations of respiratory models and of methods for measuring respiratory mechanics are examined. For each model, the physiological interpretation of the lumped mechanical parameters is summarised in terms of gas redistribution or tissue viscoelastic properties.

Implications of a biphasic two-compartment model of constant flow ventilation for the clinical setting

PURPOSE

To investigate the theoretical effects of changing frequency (f), duty cycle (D), or end-inspiratory pause length on the distribution of ventilation and compartmental pressure in a heterogeneous, two compartment pulmonary model inflated by constant flow.

METHODS

Differential equations governing compartmental volume changes were derived and solved. Validation was conducted in a mechanical lung analogue with two mechanically independent compartments. Model predictions were then generated over wide ranges of f, D, or end-inspiratory pause.

RESULTS

Disparity of compartmental end-expiratory pressure was identified as the primary mechanism by which changes in f, D, or pause alter the distribution of ventilation. Distribution of peak pressures was less sensitive to such changes. Compartmental ventilation was much less uniform than compartmental peak pressure. Ventilation could not be made entirely uniform by changes of f, D, or pause within the usual clinical range.

CONCLUSIONS

In a linear, two compartment model of the respiratory system, disparity of compartmental end-expiratory pressures is the primary mechanism by which changes of f, D, or pause alter the distribution of ventilation during inflation with constant flow. Ventilation is less evenly distributed than peak alveolar pressure, and there are limits to the beneficial effects on the distribution of ventilation to be gained from manipulations of machine settings.

Respiratory mechanics by the passive relaxation technique in conscious healthy adults and patients with restrictive respiratory disorders

The passive relaxation single-breath technique has been used primarily in anesthetized human subjects to measure total respiratory system elastance and resistance. This method was used to assess the pressure-flow characteristics in 32 relaxed, conscious patients with restrictive respiratory disorders (20 with neuromuscular disease, 12 with sarcoidosis) and 27 similarly aged control subjects free of cardiothoracic disease. Using Rohrer's pressure-flow relationship during passive expiration, P/V = K1 + K2V, considerable curvilinear pressure-flow characteristics were found in both groups. These can be attributed to a combination of the upper airway and viscoelastic and elastoplastic behavior of the respiratory system. Despite the greater elastic recoil pressure (and respiratory elastance) of the restrictive patients, their pressure-flow characteristics were similar to those of the control subjects. These findings imply structural similarities in at least the lower airways, or in the effects of retractile forces along airways compensating for reduced lung volumes.

The low-frequency dependence of respiratory system resistance and elastance in normal dogs

The resistance (R) and elastance (E) of the respiratory system were determined by fitting the equation: pressure = R x flow + E x volume to data obtained from normal anesthetized/paralyzed dogs during mechanical ventilation at different frequencies (5 to 50 breaths per min) and tidal volumes. R exhibited a 50% decrease with increasing frequency while E showed a less marked but still distinct increase with frequency. Volume-time profiles were also recorded in the same animals during passive expiration, and the frequency dependence of resistance and elastance from 0 to 1 Hz predicted from the bi-exponential curves fitted to the profiles. The way in which resistance and elastance were predicted to vary with frequency was similar to the variations determined from regular ventilation data. There were, however, some systematic differences between the actual values of resistance and elastance obtained by the two methods which may reflect certain nonlinear characteristics of the respiratory system such as static hysteresis. Nonlinearities were also evident in that both the resistances and the elastances at all frequencies showed a slight decrease with increasing tidal volume. We conclude that a large part of the mechanical behaviour of the normal canine respiratory system at low frequencies can be accounted for in terms of a two-compartment model describing a homogeneous alveolar region surrounded by viscoelastic tissue.

Respiratory mechanics determined by flow interruption during passive expiration in cats

We used the interrupter technique to measure the resistance Rinit (equal to the initial change delta Pinit in tracheal pressure divided by flow at interruption) during expiration in six normal anaesthetized-paralyzed cats. By performing interruptions at different points in expiration we found Rinit in each cat to be linearly dependent on flow. By allowing the cats to expire through two different resistances we were also able to demonstrate a volume dependence of Rinit in four of the cats. In addition, we obtained a secondary pressure change delta Pdif in each cat, as the magnitude of the slow change in tracheal pressure in the 2 sec following interruption of flow. delta Pdif was approximately constant over most of the expired volume range, and represented the difference between the static elastic recoil pressure of the respiratory system and the pressure driving flow at any volume during a passive expiration. delta Pdif became larger than delta Pinit towards the end of expiration. Since previously used methods for measuring respiratory system resistance have employed varying combinations of delta Pinit and delta Pdif as the resistive pressure drop, it is clear that measurements of resistance must be made with standard techniques under standard conditions if they are to be compared.

Evoked bronchoconstriction: testing three methods for measuring respiratory mechanics

In order to assess the usefulness of methods for noninvasive study of respiratory mechanics in intubated patients better, we studied 16 anesthetized paralyzed mechanically ventilated guinea pigs previously sensitized with ovalbumin. Respiratory system elastance and resistance were determined in control and acute antigen exposed (AAE) animals by the end-inflation occlusion method (EIOM), the single-breath method (SBM), and the interrupter technique (IT). In control group, total respiratory system resistance and elastance were constant over tidal volume range and the three methods provided similar results. In AAE group elastance was also constant throughout tidal volume and measured by SBM and IT was, respectively, 93.4 and 57.9% higher than in control group. The relationships between resistive pressure and expiratory flow became curvilinear with an upward convexity. IT underestimated both elastance and resistance measurements in AAE group. Using EIOM the determination of the homogeneous and uneven components of respiratory resistance was possible in control animals, whereas in AAE group resistance was entirely represented by its uneven component.

Evaluation of the flow-volume loop as an intra-operative monitor of respiratory mechanics in infants

Airway pressure is currently the primary indicator of respiratory mechanics used by the anesthetist in the operating room. This quantity can signal that the mechanical properties of the respiratory system have changed. However, there is a need for more sophisticated monitors of mechanics, capable of indicating the nature of the change. We have investigated the use of the tidal flow-volume loop in differentiating between an obstruction of the endotracheal tube and changes in the distribution of regional ventilation, using a computer model. Endotracheal obstruction caused the descending limb of flow-volume loop to become convex to the volume axis, whereas ventilation inhomogeneity caused the curve to become concave to the volume axis. In contrast, examination of peak airway pressure did not allow differentiation between the two conditions. We conclude that, while the peak airway pressure is useful in signaling a change in a patient's condition, the combination of airway pressure and the flow-volume loop serves as a more comprehensive monitor of respiratory mechanics.